Over the century, imidazoles have received significant attention due to their reactions and biochemical properties. Even today, research in imidazole chemistry continues. Several substituted imidazoles are known as inhibitors of P38 kinase. Therefore, it has attracted discussion on molecular structures of imidazole compounds like 2,4,5-triaryl-1H-Imidazole and its derivatives.The purpose of this study is to compare the results of theoretical calculations on structural geometry for 2,4,5-triphenyl-1H-Imidazole (TPHI) with the X-ray experiment, and survey the effects of X substitution (X=F, Cl, OH) in para position of the phenyl in 2nd position of imidazole ring on the structure of the molecule. All quantum calculations were performed using Gaussian 03W program. Geometries of TPHI and its Xsubstituted derivatives were fully optimized at B3LYP level of DFT using 6-31G**, 6-311G**, and 6-311++G** basis sets. According to the calculations at different basis sets, C6C1C7N2 dihedral angles between the plane of the rings of Ph(I) and imidazole are in the range of 6.4-7.6, 4.5-5.2 and 5.1-8.8 degrees for X=F, Cl and OH, respectively. These amounts are almost the same as that calculated for TPHI (5.8-7.4°). Alternatively, C15C10C9N2 and C21C16C8N1 dihedral angles are about 33 and 41°, respectively, for all TPHI and its mentioned derivatives. This evident difference confirms that there is only a significant resonance conjugation between imidazole and Ph(I) rings. In TPHI and its all mentioned X-substituted derivatives, the C1-C7 bond is longer than C9-C10 and C8-C16 bond lengths, that again, confirms the resonance conjugation between Ph(I) and imidazole rings. The calculated C16C8C9C10 dihedral angle is about 5.6° for TPHI which is in agreement with the experiment (4.1°). The same C16C8C9C10 dihedral angles are calculated for all X-substituted derivatives of TPHI. Also the bond angles of C2C1C7, C11C10C9 and C17C16C8 in TPHI are in excellent agreement with experimental result, and do not vary upon X substitutions. The calculated bond length of N1-H is in the range of 1.001-1.008 Å for above molecules, which is longer than experimental value, due to low scattering factors of hydrogen atoms in X-ray diffraction. The major differences between the structures of TPHI and its X-substituted derivatives are longer C3-C4 and C4-C5 bond lengths and smallerC5C4C3 bond angle in the former, because of electron withdrawing nature of X in the latter. Reference 1-R.Infante-Castillo, S.P. Hernandez-Rivera, J. Mol. Struct. 968 (2010) 6. 2-D. Yanover, M. Kaftory, Acta Cryst. E65 (2009) o711.

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